| We study the source processes of two extreme cases, the hum of the Earth and the 2004 great Sumatra-Andaman earthquake, by using long period surface waves.; To study the source mechanism of the hum, we develop an array-based method to detect and locate very weak sources of long period surface waves, utilizing the dispersive properties of Rayleigh waves. We observe the variations in seismic amplitudes at two regional arrays: BDSN (in California) and F-net (in Japan). Our results indicate that the sources of the hum are primarily in the oceans and the dominant source regions are shifting from northern Pacific to southern oceans during northern hemispheric winter and summer, respectively. The comparison of short term variations in seismic amplitudes between the arrays and the comparison of variations in seismic amplitudes to ocean wave measurements at the coasts near the two arrays indicate that the source process consists of three steps: (1) energy conversion from atmospheric perturbation (e.g., storm) to short period ocean waves, (2) non-linear interactions of short period ocean waves to generate long period ocean waves, (e.g., infragravity waves), (3) non-linear coupling of long period ocean waves to the seafloor to develop long period surface waves. In step 3, a portion of the infragravity wave can leak, propagate through the ocean, and couple to the seafloor on the other side of the ocean.; To study the very complex source process of the 2004 great Sumatra-Andaman earthquake, we jointly invert the long period (100--500 s) global seismic waveforms and near field GPS static offsets for slip distribution on the fault plane. The sensitivity test of rupture velocity indicates that the optimal rupture velocities range from 1.8 to 2.6 km/s. Our data set is not sensitive to the dip and curvature of the fault plane. We apply a Jackknife method to estimate the uncertainty in slip distribution over the given fault plane, and find that slip is well resolved along the whole rupture with uncertainties less than 23 %. |
